In controlling an alternating-current motor, a rectangular wave voltage driving method is employed in operation regions in which an output voltage is restrained by a direct-current power source voltage if a pulse width modulation (PWM) driving method is employed.
Regarding such rectangular wave control, Japanese Patent Laying-Open No. 2006-115605 (Patent Document 1) discloses a technique for detecting an amount of imbalance between ON/OFF widths of a rectangular wave voltage provided to each phase, in order to correct a timing at which a pattern of voltage waveform thereof is switched. This achieves suppression of the imbalance, which results from an error of a rotor positional sensor, between the ON/OFF widths of the rectangular wave voltage, thus allowing for a reduced offset current.    Patent Document 1: Japanese Patent Laying-Open No, 2006-115605    Patent Document 2: Japanese Patent Laying-Open No. 2006-74951    Patent Document 3: Japanese Patent Laying-Open No. 2006-14426    Patent Document 4: Japanese Patent Laying-Open No. 2001-298992    Patent Document 5: Japanese Patent Laying-Open No. 2004-23920